The purpose of improving the hydrophobicity is to impart the paper web a degree of sizing that makes the paper compatible with ink-jet printing.
The resistance properties of paper to wetting and penetration are conventionally enhanced in papermaking by means of internal sizing, where during the paper making process compounds are added into the paper pulp which increase the hydrophobicity of the paper fibres.
Printing papers, which are mainly used as office paper in various types of copiers, printers and printing machines, are expected to exhibit high brightness among other properties, as well as acceptable permanence in archive document use. The brightness and archiving permanence properties can be affected through the type of filler used for the paper. One filler compatible with the abovedescribed requirements is precipitated calcium carbonate (PCC). However, a problem is involved in the use of this filler, because it requires a neutral or alkaline environment for a proper functioning. Such a process condition excludes the use of conventional hydrophobic sizing of paper by means of the rosin-alum system. As known in the art, this drawback is overcome by the use of hydrophobic sizes based on ketene dimer compounds such as alkyl, alkenyl, aryl and alkaryl ketene dimer sizes.
Such sizes are, however, hampered by other problems particularly in paper grades intended for office printing use that have to be compatible with different printer types. Namely, besides a application in ink-jet printing, the same paper grade should do as copier paper, laser printing paper, etc. When optimized for the above-mentioned ink-jet use, the base paper must be sized with such great amounts of ketene dimer combination sizes that ultimately the size causes problems in the alternative printer types. In practice, the degree of sizing may amount to, e.g., 0.1-0.2% of fiber dry weight in the web.
Over time, ketene dimer compounds have been found problematic as a sizing agent due to their migration tendency in the finished paper. Owing to such migration, the content of the ketene dimer compound is enriched in the outer layers of the sheet. Migration is made possible by the fact that the curing reactions of ketene dimer compounds are so slow that the added agents lose their migration capability not earlier than after a few days from the finishing of the sheet.
A disadvantageous effect of ketene dimer compound enrichment is easier slippage of the sheet surface, i.e., decrease of surface frictional resistance. Reduced friction is harmful particularly in printing and copier paper grades, because the lowered threshold of slippage causes paper handling problems in printing or copying machines whose paper transfer elements fail to provide their intended function on slippery paper grades.
Also the above-mentioned filler, namely, the precipitated calcium carbonate causes indirectly easier slippage of the sheet. This is because this filler has been found to disturb the hydrophobic sizing process, whereby greater amounts of size must be used per unit weight of fiber in comparison to the use of another type of filler.
On the other hand, it has been found that ketene dimer based sizing fails to bond completely on the fiber during sheet formation, whereby a fraction of the sizing agent remains circulating in the system and or this fraction a portion bonds later on the fiber. When circulating in the system, the sizing compound is subjected to the hydrolyzing effect of water resulting in a partial decomposition of the sizing compound into corresponding ketenes. Also a fraction of the size retained in the base web will remain unbonded to the fiber, whereby the size may undergo hydrolyzation by the moisture contained in the web. These phenomena are harmful particularly in copier use, where the sheet is subjected to heating in the copying machine, whereby the decomposition of unbonded size and its migration, along with the moisture released from the sheet, to the surface of the sheet are accelerated. Resultingly, the machine parts of the copier may become contaminated and the copying result deteriorated. To eliminate these risks, determination of residual ketene content in paper grades intended for copier use has been instigated.